Conventionally, a two-layer structure with a synthetic resin membrane consisting of PTFE, etc. and a rubber membrane consisting of EPDM, FPM, etc. is known as a diaphragm attached to a diaphragm valve. (For example, see Japanese Patent No. 4232939)
FIG. 8 is a longitudinal sectional view showing an example of a diaphragm having a two-layer structure.
This diaphragm has a two-layer structure in which a rubber membrane (B) consisting of EPDM, FPM, etc. is laminated on a synthetic resin membrane (A) consisting of PTFE, etc.
An annular ridge, which functions as a guide when attached to the diaphragm valve, is provided in a vicinity of an outer edge of the synthetic resin membrane (A). This ridge consists of an upper ridge (C) protruding upward and a lower ridge (D) protruding downward from a position corresponding to the upper ridge. A lower seal ridge (F) is provided inward from the lower ridge (D) in order to secure sealing properties by being close to a valve body with a groove (E) between them.
The rubber membrane (B) is laminated inside the upper ridge (C) of the synthetic resin membrane (A), and an upper seal ridge (G) is provided on the outer edge part of the rubber membrane (B) in order to secure sealing properties by being close to a bonnet.
However, such diaphragm having a two-layer structure had a problem of the increased number of parts and increased manufacturing man-hours resulting in an increase of a manufacturing cost, compared with a diaphragm having a single-layer structure. Also, the PTFE membrane has a poor flexibility compared with the rubber membrane, thus it had a problem of a substantial reduction of sealing properties if biting of foreign substances occurs.
On the other hand, in the case of the diaphragm having a single-layer structure which consists only of a rubber material, the above-mentioned problem did not occur, but there was a problem of low durability.
For example, if the diaphragm shown in FIG. 8 has a single-layer structure which consists only of a rubber material, it is largely compression deformed when tightening and fixing a peripheral edge part of the diaphragm between the valve body and the bonnet, compared with a diaphragm having a two-layer structure with a PTFE layer. At this time, an edge of the groove, into which the upper seal ridge (G) formed on the lower surface of the bonnet fits, abuts on an inner root part (H) and an outer root part (I) of the upper seal ridge (G), so that a large stress is applied to them.
Therefore, if the diaphragm is repeatedly moved up and down for opening and closing of the flow channel, a crack will occur with these root parts (H), (I) as the starting points in a short period of time.